The present invention relates to a water cooled crucible for use in an electron beam evaporation source. In an electron beam evaporation source, an electron beam, produced by an electron beam gun, is directed at a material contained within the water cooled crucible. The electron beam melts and vaporizes the material in order to deposit the material onto a substrate. In one type of electron beam evaporation source, the electron beam gun is mounted beneath the crucible and a magnetic field produced by permanent magnets bends the electron beam through an arc of 270.degree. and into the crucible.
Electron beam guns have an output rating of anywhere from 10 to 12 kw. In order to keep the crucible from melting, water passages are provided within a supporting structure that supports the crucible. Such supporting structure can comprise a shell-like outerwall and a shell-like baffle plate coaxially spaced within the outerwall. The crucible is received within the baffle plate with an overhanging, under-stepped, rim portion of the crucible resting on the top of the outerwall. The resultant outer annular space located between the baffle plate and the outer wall form an inlet passage for the cooling water. The cooling water flows from the outer annular space through openings in the baffle plate, located near the rim of the crucible, into an inner annular space located between the crucible sidewall and the baffle plate.
Although electron beam sources are capable of processing virtually all elements or combination of elements in the periodic table, the melting and evaporating aluminum and aluminum alloys is extremely difficult due to the high thermal conductivity of the aluminum. As the electron beam impacts the top surface of the aluminum in the crucible, a certain portion of the heat is transferred to the aluminum to melt and evaporate the aluminum. Another portion of the heat is conducted through the crucible sidewall and into the cooling water. An event referred to in the art as "wetting" occurs when the aluminum and the crucible, generally fabricated from copper, alloy. In the wetting phenomenon, the aluminum melts into the crucible at a location of the crucible at or near the top surface of the molten material. Initiation of the wetting phenomenon is not fully understood. However, once initial wetting occurs between the aluminum and copper of the crucible, the thermal conductance across the interface increases. The increase in thermal conductance produces local boiling of the cooling water in the inner annular space and forms a steam pocket. Once the steam pocket forms, heat transfer between the cooling water and the region of the crucible at or near the steam pocket eventually stops. The end result of the loss of cooling causes catastrophic failure of the crucible.
The present invention provides a water cooled crucible having a greater outer surface area than water cooled crucibles of the prior art to increase heat transfer between the crucible and the cooling water at the top surface of the molten material and thereby prevent the wetting phenomenon.